Donor polarography

The reduction potentials were determined by polarography, and by CV on a hanging Hg cathode, and were irreversible, though the presence of a reverse wave was noted. Evidently, here the group 14 elements are directly affected (Table 14). The cathodic shift is understandable when L in Ph3MCo(CO)3L is changed from CO (an acceptor ligand) to the donors phosphine or phosphite. Anodic shift of the first reduction potential in the... 

Although there is a wealth of data in the literature on acid-base behavior in aprotic solvents,60 61 there are few examples of the use of buffers for polarography and voltammetry in aprotic solvents. This has occurred because most investigators have sought to keep all potential proton donors out of the system and thereby stabilize anion radicals. Although the picric acid-picrate system has been used as a buffer in a number of studies in aprotic solvents, its use in voltammetric work is limited because of the ease of reduction of picric acid. 

Owing to its stability, solubility, and highly reproducible oxidation behavior, ferrocene has long been used as an electrochemical standard in nonaqueous solvents. Not surprisingly, the electron-donor or -acceptor properties of ring substituents in ferrocenes and other metallocenes have been repeatedly evaluated with electrochemical techniques. Measurements have been obtained using polarography,150 cyclic voltammetry (CV),151 chronopotentiometry,152 photoelectron spectroscopy, 53 and Fourier transform ion cyclotron resonance mass spectrometry.154 Extensive compilations of such data are available.155 156 Historically, variations of oxidation potentials have been discussed almost solely in terms of the... 

There are indications, however, that the reduction mechanism in aprotic solvents may be different for cations in which one of the substituents possesses a-hydrogens. The reduction of a series of lO-methyl-lO-arylphenoxarsinium salts of the general structure 1 has been studied in DMF by polarography in the absence and in the presence of added proton donors. ... 

As has emerged from the foregoing discussion, conclusions may be drawn about the donor strength sequence of solvents from the equilibrium stabilities of the resulting solvates. The exact determination of these is very difficult, however, because of the very laborious and complex nature of equilibrium analysis in non-aqueous solutions. Accordingly, in many cases researchers have been satisfied with the qualitative characterization of the stabilities of the solvates formed. Of the possible electroanalytical procedures, polarography appeared the most readily utilizablc for this purpose. 

The solvents are classified from the point of view of coordination chemistry, as either acceptors or donors and reactions occurring in the solutions are related to the coordinating properties of the solvents. For this reason, the various physicochemical methods in use in the authors own laboratories, and the semi-quantitative data thereby available, have been utilized to establish the coordination forms of transition metals in various non-aqueous solutions and to correlate the results with the various solvent properties. The reader will, however, not find in the text discussion of ligand field theory, analytical applications of some of the reactions mentioned, or the electrochemistry (including polarography) of the solutions, because these aspects of non-aqueous chemistry, although of extense interest, are outside the scope of this volume. 

The role and control of acidity represents a difficult problem in nonaqueous polarography, and the presence of proton donors often changes markedly the polarographic behaviour. Extensions of the acidity scale, like Hq and H functions, have been successfully employed in some instances. 

As mentioned previously, the value determined by polarography for N-alkoxy pyridinium salts is of the order of-16 kcal/mol. This indicates that N-alkoxypyridinium salts would be expected to undergo facile reduction by photoexcited electron-rich compounds. Indeed, electron-transfer photosensitization by typical excited state electron donors such as anthracene, perylene, thioxanthene [YAG 93] and trimethoxybenzene [YAG 93] has been reported. 

The ability of phosphines to act as either one-electron acceptors or one-electron donors may be crucial in the mechanism of their cytotoxicity and allow them to interfere with electron transport processes. Phosphinium radical cations have been generated by oxidation of phosphines at a mercury anode or by y-irradiation of tertiary phosphines on silica and in sulphuric acid The oxidation potentials appear to be within the range accessible to biological systems Aryl phosphines are more easily oxidised than alkyl phosphines Ejq (polarography in CH3CN, anodic oxidation) PPha -H 50 mV, PEt3 - 415 Phosphine dimer cation radicals (R3PPR3) have been identified by... 

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